A variety of technologies are available to transmit information wirelessly, particularly digitally encoded data. A telematics unit installed in a mobile vehicle, for example, may transmit digitally encoded vehicle information when establishing communication with a telematics call center or data center. Wireless digital data, ranging from text messages to wireless file transfer, is commonly used. Digital or binary data represents meaningful information, or symbols, encoded as a sequence of logical ones and zeros. In most encoding schemes a logical one represents a “true” or “on” state, and a logical zero represents a “false” or “off” state.
Various methods are employed to transmit digital data wirelessly. These methods include Amplitude Shift Key (ASK) where ones and zeros are represented as different amplitudes at the same frequency. In this method a representation of a one may have higher amplitude than a representation of a zero. Another method is Frequency Shift Keying (FSK), which represents ones and zeros as altered frequencies with no amplitude change. In this method, a one typically has a lower frequency than a zero.
Another efficient and popular method is Phase Shift Keying (PSK) where the data to be transmitted is encoded by varying the phase representing ones and zeros. In some cases a logical one is represented by a 180 degree phase shift and a logical zero is represented by a zero degree phase shift. A variance of PSK is differential PSK, which provides a phase shift relative to the phase of the previously transmitted logical one or zero.
An unintended phase shift error may be introduced as a signal traverses through landline and wireless networks. This unintended phase shift error may cause difficulty in correctly sampling and decoding a transmitted signal.